The pituitary gland is attached by a stalk to the region in the base of the brain known as the hypothalamus. The pituitary gland has two lobes, the anterior and the posterior lobes. The posterior lobe of the pituitary gland stores and passes onto the general circulation two hormones manufactured in the hypothalamus, these being vasopressin and oxytocin. The anterior lobe of the pituitary gland secretes a number of hormones, which are complex protein or hyphenated molecules that travel through the bloodstream to various organs and which, in turn, stimulate the secretion into the bloodstream of other hormones from the peripheral organs. In particular, follicle stimulating hormone(FSH) and luteinizing hormone(LH), sometimes referred to as gonadotropins or gonadotropic hormones, are released by the pituitary gland. These hormones, in combination, regulate the functioning of the gonads to produce testosterone in the testes and progesterone and estrogen in the ovaries, and also regulate the production and maturation of gametes.
The release of a hormone by the anterior lobe of the pituitary gland usually requires a prior release of another class of hormones produced by the hypothalamus. One of the hypothalamic hormones acts as a factor that triggers the release of the gonadotropic hormones, particularly LH. The hypothalamic hormone which acts as a releasing factor for LH is referred to herein as LRF although it has also been referred to as LH--RH and as GnRH. LRF has been isolated and characterized as a decapeptide having the following structure: EQU p--Glu--His--Trp--Ser--Tyr--Gly--Leu--Arg--Pro--Gly--NH.sub.2
Peptides are compounds which contain two or more amino acids in which the carboxyl group of one acid is linked to the amino group of the other acid. The formula for LRF, as represented above, is in accordance with conventional representation of peptides where the amino group appears to the left and the carboxyl group to the right. The position of the amino acid residue is identified by numbering the amino acid residues from left to right. In the case of LRF, the hydroxyl portion of the carboxyl group of glycine has been replaced with an amino group (NH.sub.2). The abbreviations for the individual amino acid residues above are conventional and are based on the trivial name of the amino acid: where p--Glu is pyroglutamic acid, His is histidine, Trp is tryptophan, Ser is serine, Tyr is tyrosine, Gly is glycine, Leu is Leucine, Arg is arginine and Pro is proline. Except for glycine, amino acids of the peptides of the invention are of the L-configuration unless noted otherwise.
It is known that the substitution of D-amino acids for Gly in the 6-position of the LRF decapeptide can provide a peptide material having from about 1 to 35 times greater potency, relative to LRF, to effect the release of LH and other gonadotropins by the pituitary gland of mammalians. The releasing effect is obtained when the LRF analog, referred to as an LRF agonist, is introduced into the bloodstream of a mammalian.
It is also known that substitution of various amino acids for His (or the deletion of His) at the 2-position of the LRF decapeptide can produce analogs, referred to as LRF antagonists, having an inhibitory effect on the release of LH and other gonadotropins by the pituitary gland of mammalians. In particular, varying degrees of inhibition of the release of LH are obtained when His is detected (des His) or replaced by D--Ala, D--Phe or Gly. The inhibitory effect of such peptides modified at the 2-position can be further enhanced when a D-amino acid is substituted for Gly in the 6-position of the decapeptides. For example, the peptide: [des His.sup.2, D--Ala.sup.6 ] LRF, i.e., pGlu--Trp--Ser--Tyr--D--Ala--Leu--Arg--Pro--Gly--NH.sub.2, is 3 times more potent as an inhibitor for the release of gonadotropins than is the same peptide where Gly is present in the 6-position rather than D--Ala.
Some female mammalians who have no ovulatory cycle and who show no pituitary or ovarian defect begin to secrete normal amounts of the gonadotropins LH and FSH after the appropriate administration of LRF. Thus, the administration of LRF is considered suitable for the treatment of those cases of infertility where a functional defect resides in the hypothalamus. There are also reasons for desiring to prevent ovulation in female mammalians, and the administration of LRF antagonists have been used to prevent ovulation. It has also been found that the administration of potent agonists have exhibited potential use as a contraceptive by substantially decreasing the sperm count in male mammals; however, more effective methods of contraception for male mammals using LRF analogs were sought.